This invention relates generally to silicon wafer cleaning, and in particular to an apparatus and method for determining inorganic ionic contamination on a single surface of a wafer.
A substantial concern in the manufacture of silicon wafers for semiconductor chips is surface contamination. Impurities degrade wafer surfaces allowing a haze to form on the surfaces, and can cause corrosion of metallic components of semiconductor devices. Among the contaminants that have a detrimental impact on the quality and reliability of integrated circuit devices are inorganic anions such as chloride, sulfate, nitrate, and fluoride.
Accurate measurement of silicon wafer surface anion contamination is crucial to maintain quality control of the manufacturing process. Contamination levels must be monitored to ensure that products are of a quality necessary for production of integrated circuit devices. Typically, a small percentage of production wafers are randomly sampled for testing. If anion concentration exceeds an allowable level, wafer production may be halted until the contamination source(s) can be identified and eliminated.
Previous measurement methods have not allowed testing a single side of a silicon wafer. Single-side data is of primary importance since integrated circuit devices are typically mounted only on one side of the wafer, and only on that side is contamination a concern. One technique established in the art for detection of inorganic ionic contaminants on all surfaces of the wafer is to perform a water extraction, in which the wafer is immersed in pure water for a period of time to cause a transfer of anions (which are water-soluble) from the wafer surface to the water. The water is subsequently analyzed using an ultra trace analysis method such as ion chromatography (IC) or capillary electrophoresis (CE) to determine anion concentrations. This technique provides the average contamination level of the entire wafer, including both sides, but precludes measuring contamination on a single surface.
Further, previous measurement methods are subject to inaccuracy and non-reproducibility. Sampled wafers are exposed to contamination not only during the manufacturing process but also during the contamination measurement process. Airborne gas phase species in any open environment circulate past deionized water used in an extraction and contaminate it. Even class 1 cleanrooms contain airborne molecular species contaminants. When IC or CE test results indicate high levels of contamination, uncertainty arises as to whether the impurities originate in the manufacturing process or in the measurement process.